1. Field of the Invention
This invention relates to a drive circuit for a liquid crystal display panel having thin film transistors (TFT's) switching a data signal to be applied to a liquid crystal cell, and more particularly to a TFT charge characteristic compensating circuit for maintaining a constant charge characteristic of a liquid crystal cell despite changes in ambient temperature.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) panel includes liquid crystal cells, which respond to a voltage level of a data signal to control a light transmissivity, and thin film transistors (TFTs) for switching the data signal to be applied to each liquid crystal cell. The TFT's on the LCD panel have resistance values that decrease gradually as the ambient temperature increases. Also, the liquid crystal cells have a dielectric constant that increases gradually as the ambient temperature increases.
Since both the resistance values of the TFT's and the dielectric constant of the liquid crystal cells change as the ambient temperature changes, the amount of electric charge in the liquid crystal cell, via the TFT, also changes as the ambient temperature changes. This in turn causes the light transmission response of the liquid crystal cell to change with temperature as well. Thus, as the ambient temperature varies, the quality of the image displayed from the LCD panel deteriorates.
A conventional driving apparatus for an LCD panel is shown in FIG. 1. The conventional LCD panel driving apparatus includes a DC voltage converter 12, a gate line driver 14, and an LCD panel 10. The LCD panel 10 has a liquid crystal cell CLC positioned at an intersection between the a line GL and a data line DL, and a TFT MN connected among the liquid crystal cell CLC and the gate and data lines GL and DL. The liquid crystal cell CLC and the TFT MN are arranged in a matrix.
The DC voltage converter 12 supplies DC voltages required for the gate line driver 14. The DC voltage converter 12 receives a DC voltage Vd via a power input line 11 from a power supply (not shown). Also, the DC voltage converter 12 outputs a high-level gate voltage Vgh and a low-level gate voltage Vgl. The high-level gate voltage Vgh is applied, via a first resistor R1, to the gate line driver 14 and the low-level gate voltage Vgl is applied, via a second resistor R2, to the gate line driver 14 as well.
The gate line driver 14 alternates driving the gate line GL with a high level voltage and a low-level gate voltage. When the high level voltage is applied, the TFT MN turns on to apply a data signal on the data line DL to the liquid crystal cell CLC. The liquid crystal cell CLC is charged by the data signal while the TFT MN is on.
The high level voltage applied to the gate line GL is constant regardless of the ambient temperature. However, because the TFT MN in the LCD panel 10 responds differently as the ambient temperature changes, the liquid crystal cell CLC is charged differently as the temperature changes as well. As noted above, this in turn creates a changing response of the light transmission of the liquid crystal cell CLC. Accordingly, the quality of the image displayed from the LCD panel deteriorates as the ambient temperature changes.